I've used it once or twice before. The problem I have with it is the crediting clause in the creative commons license (see below), unless I'm misunderstanding it. Let's say I use 10 sounds for a film, that's a big chunk of credit space :-(. Getting a producer or product manager to agree to significantly multiply the credit space they give to the sound department for a few particular sound effects ain't easy.
Attribution and Notice.
If You distribute, publicly display, publicly perform, or publicly digitally perform the Work or any Derivative Works or Collective Works, You must keep intact all copyright notices for the Work and give the Original Author credit reasonable to the medium or means You are utilizing by conveying the name (or pseudonym if applicable) of the Original Author if supplied; the title of the Work if supplied; to the extent reasonably practicable, provide the Uniform Resource Identifier, if any, that Licensor specifies to be associated with the Work or a Derivative Work, unless such Uniform Resource Identifier does not refer to the copyright notice or licensing information for the Work; and in the case of a Derivative Work, provide a credit identifying the use of the Work in the Derivative Work (e.g., "Remix of the Work by Original Author," or "Inclusion of a portion of the Work by Original Author in collage"). Such credit may be implemented in any reasonable manner; provided, however, that in the case of a Derivative Work or Collective Work, at a minimum such credit will appear where any other comparable authorship credit appears and in a manner at least as prominent as such other comparable authorship credit.